Conventional electrophotographic copying and printing forms duplex copies or prints in one or two ways. The large majority place a toner image on one side of a sheet of paper, fuse the image, and then recirculate the paper back through the system to receive a second toner image on the opposite side which is also fused along with further fusing of the first toner image. Some of the most difficult paper handling problems associated with modern copiers and printers occur in the substantial additional handling associated with this approach. Further, one toner image receives substantially more fusing than the other which restricts the materials usable in the system.
In the second approach, two transfer stations are used in tandem with the receiving sheet first receiving a toner image at one transfer station then being immediately turned over and fed to the second transfer station to take the second toner image. The receiving sheet carrying both toner images is then fed to a duplex fuser where the toner images are fused simultaneously. This approach, called "single-pass duplexing", is shown in a number of references, for example, U.S. Pat. No. 4,191,465, issued Mar. 4, 1980, to Boase et al and U.S. Pat. No. 4,194,829, issued Mar. 25, 1980, to Cavagnaro.
U.S. Pat. No. 4,714,939, issued Dec. 22, 1987 to D. K. Ahern et al, is one of a number of references which show the use of an intermediate transfer member to form duplex toner images on a receiving sheet. According to that patent, consecutive toner images are created electrophotographically on a photoconductive image member. The first toner image is transferred to an intermediate member, which can be a roller or an endless belt. A receiving sheet is fed between the photoconductive image member and the intermediate member and the first image is transferred to one side of the sheet and the second image is transferred to the other side of the sheet.
This system can also be used to create color images by transferring two or more images in registration to the intermediate with the combination being transferred back to the first side of the sheet.
These systems have the great advantage of a straight relatively short path for the receiving sheet from its supply to the fuser. Unfortunately, one image is transferred one more time than the other image. This forces a change in the exposure station between images. The most common approach to solving this problem is to add an extra mirror in the optical path for only one of the two exposures.
U.S. Pat. No. 4,688,925, issued Aug. 25, 1987 to R. A. Randall, shows a system with two intermediates in which a first image is transferred to one intermediate and a second image to the second. A receiving sheet is fed between the intermediates to receive the images on opposite sides. This cures the problem of reversing optics but requires two articulating intermediates.